The present invention is directed toward an improved plastic package for a semiconductor integrated circuit device and towards leadframes and methods for making such a package.
Semiconductor integrated circuit devices, which are typically formed from silicon or gallium arsenide, are conventionally enclosed in plastic packages. The elements of such a package include a metal leadframe, an integrated circuit device, bonding material to attach the integrated circuit device to the leadframe, bond wires or other connectors that conductively connect metal pads on the integrated circuit device to individual leads of the leadframe, and a hard plastic encapsulant material that covers the other components and forms the exterior of the package.
FIG. 1 is a conventional metal leadframe 10. Although not shown, several identical leadframes 10 are formed in an array from a single sheet of metal and each leadframe of the array is processed in parallel.
Leadframe 10 includes a central metal die pad 11 (shown by horizontal hatching) and radiating metal leads 12 adjacent to die pad 11. An opposite end of each lead 12 is connected to the peripheral frame of the leadframe (not shown). A dam bar (not shown) also crosses and connects the leads. The frame and dam bar are removed during a trim and form step. The portion of each lead 12 that is within the dashed line is plated with silver. The perimeter of die pad 11 also may be plated. A ring of an adhesive polyimide tape 13 (shown by diagonal hatching) crosses and connects each of the leads 12 at a midpoint and serves to stabilize the leads during processing. Four tie bars 14 support die pad 11 and connect it to the peripheral dam bar and frame (not shown). Five leads 15 are connected to die pad 11 for support. Die pad 11 is downset, as indicated by downset marks 16 on tie bars 14 and leads 15.
An alternative prior art embodiment of a die pad shown is in FIG. 2. Metal die pad 20 is surrounded by a ring 21. Ring 21 is connected by sixteen metal connectors 22 to die pad 20. Tie bars 23 connect die pad 20 to the remainder of the peripheral frame of the leadframe (not shown). This configuration facilitates spacing of the leads (not shown) and provides an encapsulant locking feature.
In a completed package, the integrated circuit device is mounted on the die pad. Typically, integrated circuit devices require connection to a power voltage source, sometimes denoted xe2x80x9cVdd,xe2x80x9d and a ground voltage source, sometimes denoted xe2x80x9cVss.xe2x80x9d A lead of the package typically is dedicated to supply Vdd. Sometimes, a second dedicated lead provides Vss. The use of dedicated leads for power and ground voltages, however, limits the flexibility of the package because the user of the package, for example, a chip manufacturer or computer manufacturer, cannot vary which leads are to be used for connection to input and output (xe2x80x9cI/Oxe2x80x9d) signals and which leads are to be used for connection to Vdd and Vss. Sometimes, a second dedicated lead provides Vss. The use of dedicated leads for power and ground voltages, however, limits the flexibility of the package because the user of the package, for example, a chip manufacturer or computer manufacturer, cannot vary which leads are to be used for connection to input and output (xe2x80x9cI/Oxe2x80x9d) signals and which leads are to be used for connection to Vdd and Vss.
The present invention includes a package made from a leadframe having an inventive design. The package overcomes the limitations of prior art packages by allowing all leads of the package to be used for either I/O signals or power or ground leads. This allows a common package design to be used even when the location of the power voltage, ground voltage, and I/O signal inputs vary among different electronic devices.
A leadframe within the present invention includes a central metal die pad. A metal ring surrounds the die pad. The metal ring is spaced a small distance apart from the die pad. A nonconductive connector is attached between the die pad and the surrounding ring and holds the die pad and the ring together. An example nonconductive connector is a polyimide adhesive tape. A plurality of leads extend from the periphery of the leadframe toward the die pad and ring. The leads terminate adjacent to the ring. The ring is between the ends of the leads and the die pad. None of the leads directly connect to the ring. In an alternative embodiment, one or more of the leads is directly connected to the ring and provides support for the ring. Depending on the application of the package, a bypass capacitor or decoupling capacitor is connected between the die pad and the ring and filters the input power voltage.
The present invention also includes a method for making the leadframe described above. Step 1 of the method provides a metal sheet. Step 2 patterns the metal sheet to form the die pad, the surrounding ring, and the leads. At this initial stage, a temporary metal connector or a plurality of temporary metal connectors also are formed from the metal sheet and connect the die pad to the ring. Step 3 connects a nonconductive connector, such as polyimide adhesive tape, between the die pad and ring. A plurality of nonconductive connectors, such as two or four segments of tape, may be used to connect the die pad and ring. Step 4 removes the temporary metal connector(s) so that only the nonconductive connector(s) holds the die pad to the surrounding ring. Step 4 can be accomplished using several methods, including punching, etching, or laser cutting methods. Optionally, the tips of the leads and the periphery of the die pad are plated with another metal, such as silver, to facilitate connection to bond wires. Further, a bypass, decoupling, or filter capacitor may be connected between the die pad and the surrounding ring. Capacitor connection may be accomplished using a variety of methods, such as conductive adhesive or soft soldering methods.
A package made from the above-described leadframe includes the metal die pad, surrounding metal ring, nonconductive connector(s), and leads. An integrated circuit device is on the die pad. The leads are connected by conductors, e.g., bond wires, to conductive pads on the integrated circuit device. One of the leads is connected by a bond wire to the ring. Typically, the ring is connected to the lead that is attached to a power voltage input Vdd. The ring in turn is connected by a bond wire to a pad on the integrated circuit device for inputting the power voltage to the integrated circuit device. The die pad is isolated from the ring and is floating. In an alternative embodiment, a lead that attaches to a ground voltage is connected by a bond wire to the die pad. A capacitor (filter, bypass, or decoupling) may be connected between the ring and the die pad. Hardened encapsulant material covers the entire structure, including the nonconductive connectors and the integrated circuit device.
A method of making the above package also is within the present invention. Step 1 of the method provides a leadframe having the features described above, including a central die pad, a surrounding ring separated from the die pad, a nonconductive connector holding the die pad to the ring, and radiating leads. Step 2 attaches an integrated circuit device to the die pad. Step 2 is accomplished using industry standard practices utilizing conductive or nonconductive organic epoxies or polyimides, as well as soft solders and conductive adhesive tapes. Step 3 installs conductive connectors between the integrated circuit device and the leads, between one or more of the leads and the ring, and between the ring and the integrated circuit device. Step 3 is accomplished using industry standard practices, such as wire bonding using thermosonic, thermo-compression, or ultrasonic wedge methods. Step 4 applies an encapsulant material over the entire structure. Typically, industry standard thermoset epoxies are used. These include Epoxidized ortho uresol novolac (xe2x80x9cEOCNxe2x80x9d), bi-phenyl (xe2x80x9cEpxe2x80x9d), dicyclopentadiene (xe2x80x9cDCPDxe2x80x9d), and multi-functional (xe2x80x9cMFxe2x80x9d) epoxies. Step 5 hardens the encapsulant material. Finally, Step 6 trims and forms the leads.
The following detailed description will further describe aspects of the present invention.